The present invention relates generally to methods and apparatus for measuring blood oxygen saturation in a retinal vessel and, more particularly, to methods and apparatus for measuring blood oxygen saturation in a retinal vessel based upon the separate detection of optical signals that traverse the retinal vessel a single time, thereby obtaining well-calibrated measurements of the blood oxygen saturation of the blood within the retinal vessel. In this regard, the method and apparatus of the present invention advantageously separate optical signals that pass through the retinal vessel multiple times, optical signals that reflect from the retinal vessel and optical signals that are backscattered by red blood cells within the retinal vessel from optical signals that traverse the retinal vessel a single time such that the optical signals that traverse the retinal vessel a single time can be separately detected and processed, with the other optical signals being either discarded or similarly separately detected and processed.
A variety of spectroscopic oximetry techniques have been developed to monitor the blood oxygen saturation and blood oxygen content in retinal vessels. By monitoring the blood oxygen saturation, the arteriovenous oxygen difference can be determined as described by U.S. Pat. No. 5,308,919 to Thomas E. Minnich, U.S. Pat. No. 5,776,060 to Matthew H. Smith, et al., and U.S. Pat. No. 5,935,076 to Matthew H. Smith, et al. Based upon the arteriovenous oxygen difference, the cardiac output of a subject can be determined in order to assist in post-operative monitoring and the management of critically ill patients. By monitoring the blood oxygen saturation, the loss of blood can be detected, and the rate and quantity of blood loss over time can be estimated as described by U.S. Pat. No. 5,119,814 to Thomas E. Minnich.
In addition to the variety of invasive techniques that require blood to be drawn, oftentimes repeatedly, from a patient, a number of non-invasive spectroscopic oximetry techniques have been developed to measure the blood oxygen saturation of a patient without requiring blood to be drawn from the patient. For example, a number of noninvasive spectroscopic oximetry techniques have been developed which measure the blood oxygen saturation of a patient based upon the transmittance of the blood within a retinal vessel, such as a retinal vein or a retinal artery. For example, U.S. Pat. Nos. 5,776,060 and 5,935,076 describe techniques for measuring the oxygen saturation of blood within a retinal vessel by illuminating the retinal vessel with light having a combination of wavelengths and then measuring the transmittance of the blood within the retinal vessel in response to the illumination at each of the selected wavelengths. Based upon the respective transmittance of the blood within the retinal vessel that is measured at each of the selected wavelengths, the oxygen saturation of the blood within the retinal vessel can be determined. The contents of U.S. Pat. Nos. 5,776,060 and 5,935,076 are hereby incorporated by reference in their entirety.
As will be apparent, the light with which a retinal vessel is illuminated can be reflected and transmitted in a variety of different manners. For example, some of the light will be immediately reflected by the retinal vessel, while other portions of the light will be backscattered by the red blood cells within the retinal vessel. Other portions of the light, termed xe2x80x9cdouble pass lightxe2x80x9d, will pass through the retinal vessel, be reflected from the retinal and/or choroidal layers and again pass through the retinal vessel, thereby traversing the retinal vessel twice. Further, some portion of the light, termed xe2x80x9csingle pass lightxe2x80x9d, will pass through the retinal vessel, diffuse laterally through the retinal and/or choroidal layers and then exit the pupil without again traversing the retinal vessel.
Regardless of the particular paths traveled by the optical signals, the optical signals that return from the eye are collected by a detector and an associated processing element, such as a microprocessor, a personal computer or the like, can determine the blood oxygen saturation within the retinal vessel based upon the light that is returned. In order to determine the blood oxygen saturation, techniques have been developed to account for light that has been reflected and/or transmitted in each of the various manners described above. As a result of the variety of different ways in which light can be reflected and/or transmitted, however, the equations that must be solved to determine the blood oxygen saturation within the retinal vessel are quite complicated and may reduce the accuracy with which the blood oxygen saturation can be determined.
In this regard, the single pass light contains information relevant to determining the oxygen saturation of the blood in the retinal vessel. However, the light that is returned from the eye and is detected includes not only the single pass light but also light that has propagated along other paths, such as light that is reflected from the retinal vessel, light that has been back scattered by the red blood cells within a retinal vessel and double pass light. While the light that has propagated along these other paths is helpful in creating a visual image of the retinal vessel, the light that has propagated along these other paths is generally less valuable for purposes of determining the oxygen saturation of the blood within the retinal vessel than the single pass light.
While the separation of the single pass light from the other components of the returning light would simplify the equations that must be solved to determine the blood oxygen saturation and improve the accuracy with which the blood oxygen saturation can be determined, it has heretofore been difficult to separate the single pass light that contains the information of merit from the light that has propagated along other paths. Thus, while advantageous non-invasive spectroscopic oximetry techniques have been developed, it would be desirable to improve these techniques in order to more accurately determine the oxygen saturation of the blood within a retinal vessel.
A method and apparatus are provided for more accurately measuring the blood oxygen saturation with a retinal vessel. In this regard, the method and apparatus separate the single pass optical signals that have only traversed the retinal vessel once from the other optical signals. By separately detecting and analyzing the single pass optical signals, the method and apparatus can measure the blood oxygen saturation within the retinal vessel in a more straightforward manner utilizing simplified equations, thereby permitting a more accurate measurement of the blood oxygen saturation.
According to one aspect of the present invention, the apparatus generally includes an optical source for illuminating the retinal vessel with optical signals. The apparatus also includes a filter disposed within the path of the optical signals returning from the eye. The filter is capable of separating the single pass optical signals from the other optical signals that return from the eye. The filter may be disposed at a focal point of the optical signals returning from the eye to block or otherwise redirect the other optical signals, while permitting the single pass optical signals to pass. In one embodiment, the filter is an aperture that has a central stop and a transmissive portion at least partially surrounding the stop to only permit some optical signals to pass. As such, the aperture preferentially passes single pass optical signals that have diffused through the retinal layer and/or the choroidal layer of the eye while traversing the retinal vessel only once. The aperture can have a variety of designs. For example, the aperture can be an annulus, an anti-pinhole, a slit-annulus or an anti-slit. Regardless of the configuration, the central stop of one embodiment of the aperture is at least partially reflective to selectively redirect the other optical signals.
The apparatus also includes a detector for detecting at least the single pass optical signals that have been separated from the other optical signals by the filter. In one embodiment, the detector may also separately detect the other optical signals that have been separated from the single pass optical signals by the filter. The apparatus may further include a processing element for determining the blood oxygen saturation within the retinal vessel based upon at least the single pass optical signals. In the embodiment in which the detector also separately detects the other optical signals, the processing element can determine the blood oxygen saturation within the retinal vessel based upon the separate contributions of the single pass optical signals and the other optical signals. In either instance, the processing element can determine the blood oxygen saturation within the retinal vessel based upon a simplified set of equations since the single pass optical signals have been separated from the other optical signals, thereby permitting increased accuracy in the determination of the blood oxygen saturation within the retinal vessel.
According to another aspect of the present invention, a method for separately detecting single pass optical signals is provided. According to this method, a retinal vessel within the eye is illuminated with optical signals. The single pass optical signals that return from the eye are separated from the other optical signals and detected. In one embodiment, the other optical signals are redirected and may also be detected, albeit separately from the single pass optical signals. In order to separate the single pass optical signals from the other optical signals, the optical signals returning from the eye that are focused upon a stop, i.e., the other optical signals, may be blocked, while passing the optical signals that are offset from the stop, i.e., the single pass optical signals. Regardless of the manner in which the single pass optical signals are separated from the other optical signals, the blood oxygen saturation within the retinal vessel may be determined based upon the single pass optical signals and, in some embodiments, the other optical signals that have been detected.
According to the present invention, the single pass optical signals are separated from the other optical signals, such as the optical signals that have reflected from the retinal vessel and that have been backscattered by the red blood cells within the retinal vessel as well as the double pass optical signals. By separating the single pass optical signals from the other optical signals, the method and apparatus need not attempt to solve the complex equations that are otherwise required to determine the blood oxygen saturation in instances in which all of the returning optical signals are detected as a whole. Instead, the method and apparatus of the present invention can utilize equations that define the blood oxygen saturation based upon the detected optical signals that are greatly simplified and the blood oxygen saturation values that are determined according to the method and apparatus of the present invention may therefore have increased accuracy relative to conventional techniques.